1. Field of the Invention
The present invention generally relates to nuclear fuel assemblies for use in nuclear reactors, and more particularly to an upper tie plate/reaction pin disengaging feature for a pressurized water reactor nuclear fuel assembly to prevent the unintentional removal of a fuel assembly from the reactor core.
2. Background
Nuclear reactors need to be periodically maintained and inspected as well as refueled. Refueling operations involve the removal of the equipment and apparatus positioned in the reactor vessel above the reactor core which comprises nuclear fuel assemblies each of which contain nuclear fuel rods. Some of the equipment which needs to be removed during refueling operations include the upper core internals and support structure, the lowermost portion of which includes the upper core support plate. During the refueling operations, the upper core support structure including the upper core plate is removed, the refueling and related fuel operations are performed, and the upper core plate is then reinstalled.
The upper core support plate serves to align the upper portion of the fuel assemblies which are positioned at their lower ends in predetermined positions in the lower core support plate so that the fuel assemblies are parallel to one another. Extending downward from the underside of the upper core plate are fuel assembly alignment pins two of which insert into two alignment holes in the upper tie plate of each of the fuel assemblies. As the upper core support plate is aligned over the top of the reactor core and lowered onto the fuel assemblies, the alignment pins slide into their respective alignment holes and align the upper portion of the fuel assemblies into their predetermined positions. FIG. 2 depicts two of the alignment pins of this upper core support plate properly disposed through the alignment holes in the upper tie plate of a fuel assembly.
If misalignment occurs between one or both of the alignment pins which are to be inserted into each fuel assembly, and the fuel assembly, then the bent alignment pin(s) can fail to be inserted into its respective alignment hole in the upper tie plate and become wedged against the upper tie plate, or can be inserted into and become wedged into its respective alignment hole in the upper tie plate. (FIG. 3).
Such misalignment, bending and wedging of the alignment pins in the alignment hole(s) in the upper tie plate or against the upper tie plate of the fuel assembly can go undetected since the fuel assemblies could nonetheless be in their parallel and proper positions within the reactor vessel (not shown). Potential resulting problems from the wedging of the alignment pin in alignment hole of the upper tie plate of the nuclear fuel assembly will usually not be detected until the next refueling or maintenance operation when the upper core support plate will again be removed. One of the potential resulting problems which can and does occur is the unintentional lifting of a fuel assembly.
At the next refueling operation, the upper core support plate would be lifted during preparations for defueling, and the irradiated fuel assembly which had become wedged and stuck to the upper core support plate because of the bent alignment pin(s) would also be lifted.
An unintentional lifting and unsecured removal of an irradiated nuclear fuel assembly from the reactor core in this manner would at least result in substantial delay and inconvenience in the refueling operations until the stuck fuel assembly is safely freed and secured. More importantly however, is the safety and health consequences of the potential release of fission products from within the fuel rods as a consequence of dropping the unintentionally lifted fuel assembly, perhaps onto other irradiated nuclear fuel assemblies, and breaching the fuel rod cladding. Since the primary coolant system is not a closed system during reactor refueling operations, the release of fission products would not thereby be entrained in the primary coolant system. Accordingly, the fission products would be released into the containment building.
The unintentional lifting of a fuel assembly when it is stuck or jammed with the alignment pin of the upper core support plate could go undetected if the weight of the stuck fuel assembly(ies) together with the weight of the upper core plate and upper core internals is within the maximum allowable weight for the upper core plate and upper core internals. The unintentional lifting of the fuel assembly could furthermore go undetected despite the adherence to other procedural or operational limitations. Similarly, the use of underwater lighting and cameras are susceptible to equipment failure and/or operator error and their installation and use do not ensure detection of the wedged pin or the lifting of the fuel assembly.
It would thus be an advantage if the unintentional lifting of a fuel assembly could be eliminated.
It would be a further advantage if the unintentional lifting and removal of a fuel assembly from the reactor core or vessel due to a wedged alignment pin of the upper core plate could be avoided.
It would thus be a further advantage if the unintentional lifting, removal and possible dropping a stuck fuel assembly could be eliminated.